1. Field of the Invention
The present invention relates to an ultrasonic motor.
2. Description of the Related Art
Japanese Patent Application Laid-open No. Hei9-117168, for example, discloses an ultrasonic motor that generates an elliptical vibration by combining a longitudinal vibration and a torsional vibration, and rotation drives a rotor based on the generated elliptical vibration. FIG. 1 of Japanese Patent Application Laid-open No. Hei9-117168 depicts an exploded perspective view of a vibrator. The vibrator has a structure in which a plurality of piezoelectric elements is arranged between elastic bodies that are cut obliquely with respect to an axis of the vibrator. Positive electrodes of the piezoelectric elements are divided into two groups. These groups will be called Phase A and Phase B electrodes in the current specification.
The longitudinal vibration can be generated in a bar-shaped vibrator by applying alternating voltages of the same phase to both Phase A and Phase B electrodes. On the other hand, the torsional vibration can be generated in the bar-shaped vibrator by applying alternating voltages of opposite phases to both Phase A and Phase B electrodes. A position of a groove in the vibrator is adjusted such that a resonance frequency of the longitudinal vibration and a resonance frequency of the torsional vibration substantially match. When alternating voltages that differ by π/2 phase are applied to Phase A and Phase B electrodes, the longitudinal vibration and the torsional vibration are generated simultaneously, thereby generating an elliptical vibration on a top surface of a bar-shaped elastic body. In this state, by pressing the rotor on the top surface of the bar-shaped elastic body, the rotor can be rotated in a clockwise direction (CW direction) or a counterclockwise direction (COW direction).
The ultrasonic motor disclosed in Japanese Patent Application Laid-open No. Hei9-117168 has various drawbacks. For example, as shown in FIG. 1 of that application, both the piezoelectric element and the elastic body are necessary, the elastic body must be cut obliquely, and the groove must be made in a portion of the elastic body to match the resonance frequencies of the longitudinal vibration and the torsional vibration. Thus, in the ultrasonic motor, the overall structure of the conventional vibrator is very complicated and an efficiency of generation of a torsional resonance vibration is low.